I have an 01 Elantra with an automatic transmission. I got gas yesterday and after filling up, on my way home the transmission felt a little funny. All of a sudden the transmission made a kicing feeling with a thumping sound and then the check engine light came on. The transmission then refused to up or down shift, I think getting stuck in 3rd gear. I say this because from a stop the car was very sluggish, indicating it was not in first gear. Even If i manually put the transmission in L,2, or 3, it felt exactly the same. After shutting the car on and off a couple of times the check engine light went off and then the car shifted perfectly. What ever is causing the check engine light to come on is some how causing the transmission not to shift. I drove the car around last night to test it out and it seemed to happen randomly several times, sometimes requiring frequent restarts to clear the ecu. I thought this might have been releated to the gas that I just got, since I have never had a problem like this before. I have put a bottle of dry gas in and have used a quarter tank of gas so far and the check engin light or shifting problem as not returned. Any ideas what this could be? If the check engine light comes back on I'm going to go to autozone to have the code pulled.